The invention relates to a flame detector type of fire alarm with a circuit which, by photoelectric means and a band-pass filter, receives the emission of a flame at least in the wavelength range of carbon dioxide and at the flicker frequency range of the flame and produces useful signals therefrom for an alarm means.
It is generally known that most flammable substances such as wood, petroleum, oil and hydrocarbons or carbohydrates--in short, organic materials--emit strongly in the wavelength ranges of approximately .lambda.=2.7 .mu.m (micrometers) and particularly at approximately .lambda.=4.4 .mu.m when they burn. Radiation emission takes place in line spectra and band spectra, the wavelength range 2.7 .mu.m being characteristic for both water and carbon dioxide and 4.3 .mu.m being a characteristic of only carbon dioxide. The article entitled "Fire Detection using Infrared Resonance Radiation", pages 55 to 60, FIG. 6 which appeared in the journal "Report of Fire Research Institute of Japan", Ser. No. 30 of December 1969 describes the circuit of an alarm which is sensitive to flame emission and temperature. This alarm is designed for the infrared range. However, it is not false alarm-proof. If spurious infrared radiation is present, e.g. radiators or ovens, whose thermal radiation is periodically interrupted by an intervening fan or the like in a particular rhythm, an undesired alarm signal can result although there is no fire or flame.
French Patent No. 2 151 148 evaluates two wavelength ranges or wavebands for giving alarms in the case of fire. Selectivity results from the arrangement of two narrow-band optical filters which only transmit for the two wavelength ranges .lambda.=2.7 and .lambda.=4.3 .mu.m. The photoelectric voltages produced by these two wavelength ranges are evaluated for giving the fire alarm. However, as tests have shown, this alarm tends to give false alarms in the case of spurious radiation sources of suitable colour temperature, so that the false alarm rate cannot be effectively reduced with this alarm.
The object of the present invention is to substantially reduce the false alarm rate of a fire alarm so that, despite the occurrence of interference sources, the alarm clearly recognises each flame or fire as such and gives the necessary alarm signal.